An annual report on the development of China's fitness and sports club industry shows a sharp increase in the number of gyms over the past year.

However, the report warns that some gyms are not being manned by qualified trainers.

Completed by the Capital University of Physical Education and Sports and released in Beijing on Tuesday, the report also revealed that some yoga and pilates clubs have been employing PE teachers or members, even, to work as uncertified trainers, which in some cases have resulted in clients being hurt.

Zhong Bingshu, president of the university, said that more than 70 percent of fitness trainers in China are aged between 20 and 30.

But few had received professional training from universities, despite various certificates being available for sports trainers to take in China.

"Without a national standard, it is hard to make sure that all clubs are using qualified trainers," Zhong said.

According to the report, more than half of the country's gym clubs are charging an annual membership of 3,000 to 4,000 yuan or above. But if customers aren't satisfied with their service once they have paid, it remains difficult to a refund.

The report didn't indicate how many gyms they interviewed.